To avoid the heat generated by incandescent lamps during usage, the decorative lamp string of the present invention utilizes an LED lamp to replace the conventional tungsten lamp, which provides utility and reduces the temperature of the lamps, saves energy and overcomes conventional difficulties. However, the LED lamp has electrodes with defined polarities, i.e. while assembling, locations of the connected plus and negative poles have to be correct, otherwise the lamp will not provide illumination.